[2020] Barely any zombies roaming around will more spawn or do I need to mod it?

You could mod it, or find an existing mod that makes this change (see the Modding sub-forum). But you might instead go to the wasteland biome, where there's no rest period after clearing the roaming zombies before they appear again.

Originally posted by mONONOke_____: is there a reason for so few zombies outside? perfomance or whatever?
its little bit sad.
zombie count should raise with higher gamestages.

A reason cited has been that POIs are now packed with sleeper zombies, which occupy many of the limited slots for simultaneous active zombies. And yes, the limit is for performance. Sleepers were added in what, A15 I think? And that’s when the biome zombies were dramatically reduced.

That didn’t stop them from dramatically increasing animal spawns in A19 though, so hopefully that’s a sign they were being conservative and can raise biome spawns again. Though the general strategy will still have to apply: tougher zombies at higher gamestages rather than more zombies at higher gamestages.